The image of an illuminated square is obtained on a screen with the help of a converging lens. The distance of the square from the lens is 40 cm . The area of the image is 9 times that of the square. The focal length of the lens is :

Step-by-step solution

If side of object square = and side of image square = ^ From question, ^ 2 =9 or ^ =3 i.e., magnification m =3 u =-40 ~cm v =3 40=120 ~cm f =? From formula, 1 v - 1 u = 1 f aligned & 1 120 - 1 -40 = 1 f & or, 1 f = 1 120 + 1 40 = 1+3 120 f =30 ~cm aligned